Fibrous media containing millimicron-sized particulates

ABSTRACT

A self supporting fibrous matrix containing immobilized therein at least about 5% by weight of micro-particulate, with an average diameter less than 1 micron, preferably fumed silica, or alumina, and flocculating amounts of an organic polycationic resin and an organic polyanionic resin, is useful for fluid treatment and filtration processes, especially delipidization and depyrogenation of fluids such as serum.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to fibrous media containing millimicronsized particulates, especially media containing fumed silica or fumedalumina.

2. Brief Description of the Prior Art

The technique of flocculating negatively charged filter aid particulatesor adsorbents and fibers, by means of positively charged polymers is acommon practice in the production of filtration media.

For example, Malcolm, U.S. Pat. No. 3,647,684 describes a felted fibrousmatrix containing silicic acid (hydrated silica) interdispersed therein,useful for thin layer chromatography wherein the silicic acid hasparticle sizes within 1 to 10 microns, and which contains a cationicmaterial in concentrations no higher than 1.5%. Ostreicher, U.S. Pat.Nos. 4,007,113 and 4,007,114 describes a matrix of self-bonding fiberscontaining interdispersed therein a particulate filter material, thesurface of which is modified with an organic colloid. Additional patentsrelating to fibrous media containing interdispersed particulate materialare: Pall et al, U.S. Pat. No. 3,573,158, Leifield, U.S. Pat. No.3,455,818 and Pall et al, U.S. Pat. No. 3,238,056.

The application of positive charge flocculation techniques for theformation of specialized and improved media has also been described incommonly assigned copending U.S. patent applications Ser. No. 164,797filed May 30, 1980, Ser. No. 147,975 filed May 8, 1980, now U.S. Pat.No. 4,305,782, and Ser. No. 123,467 filed Feb. 21, 1980, now U.S. Pat.No. 4,309,247, which are herein incorporated by reference.

The porous filter media described in these applications are comprised offiber-particulate and fiber-fiber mixtures, with cationic chargemodifiers serving as charge modifiers, wet strength providers and alsoas flocculating or dispersion agents in the forming slurry system.

The mixtures are formed dynamically into sheets by vacuum felting froman aqueous slurry, and subsequently drying the finished sheets as thefinal product. The rate of production of such porous filter media isgoverned by the porosity of the sheets to be produced. A highly openfilter media comprising particle sizes of 50 microns or larger requiresonly a few seconds to be felted, whereas a tight media utilizingparticle sizes of 1-2 microns or smaller would require more than 5minutes to be felted. Sometimes, media containing the finest grades ofparticulate additives cannot be felted at all due to improperflocculation. Therefore, it is impossible to form a fibrous media withadsorbent particles less than 1 micron by current techniques. Moreover,the retention of such small size particles in the matrix structure is aserious problem. Most of them are lost to the water drainage duringfelting.

The limitations and drawbacks involved in current fibrous mediamanufacturing processes can be understood from the following method offormation. The application of vacuum for the formation of fibrous mediais predominantly a hydromechanical process. A slurry containing all thecomponents drains through a 100 mesh wire screen perpendicular to theplane of the screen, and drags all the components with its movementduring felting. Large fibers which have the largest surface area incontact with water of any of the components in the slurry, receive thestrongest viscous drag force and settle ahead of others to form a bottomfibrous network. This process provides a self-adjusting mechanism foruniform distribution of particles and fibers in the fibrous structure,based upon the fact that the drainage flow always seeks the path ofleast resistance. While the large fibers are preferentially retainedduring the initial deposition, smaller fiber fragments tend to migratethrough the fiber mat of long fibers and become lodged in the interfiberholes, to provide a coherent mat structure for adsorbent particles tosettle in. Continuous application of vacuum after sheet forming, inducesmat compaction. The compacting force exerted by the high vacuum, furthersqueezes water molecules out of the wet pad, and forces adsorbentparticles closer together to form a pad with definite porosity.

The above described method, however, is applicable only to particleslarger than about 1 micron. In the case of particle sizes less than 1micron, especially millimicron sized ones, such particles either fail tobe retained by the fiber matrix, or fail to be further felted afterforming a thin layer of compact particulate.

In particular, the formation of fibrous mats with a high load ofmillimicron sized particulate (e.g., higher than about 30% by weight)and with high porosity is near to impossible using the feltingtechniques of the prior art. Among the millimicron sized particles ofgreat interest for commercial chemical and biochemical applications isfumed silica.

The removal of lipids through adsorption on silica is a common practicein chemistry and biochemistry. See e.g. Stephan, U.S. Pat. No. 3,686,395and commonly asigned copending U.S. patent application Ser. No. 238,686filed Feb. 27, 1981 to Carpenter and Cone entitled TISSUE CULTURE MEDIA.The efficiency of lipid removal by silica differs with the process bywhich the silica is made. Silicas precipitated from the vapor phase arebetter lipid removal agents than those precipitated from sodium silicatesolutions. Typical commercial products of silicas made from vapor phaseare Cab-O-Sil®, Aerosil® (Degussa) or Sipernet 22S®. These productsexist in minute particles having average diameters from 7 to 18millimicrons (or nano-meters). They are produced by the hydrolysis ofsilicon tetrachloride vapor in a flame of hydrogen and oxygen. At aflame temperature of 1270° K. the vapor pressure of SiO₂ is only 10⁻⁸Torr, so that there is a very high supersaturation, resulting in largenumbers of small nuclei forming silica spheres with diameters rangingfrom 7 to 18 millimicrons on the average. These molten spheres, termedprimary particles, collide and fuse with one another to form branched,three dimensional, chainlike aggregates. During the hydration of fumedsilica, hydroxyl groups become attached to some of the silicon atoms onthe particle surfaces. This makes the fumed silica surface hydrophilic,and capable of hydrogen bonding with other molecules.

Fumed silica is fluffy and low in density (approximately 2 lbs. per cu.ft.). Even a small amount of fumed silica packed in a column will createextremely high pressures upon contact with buffer solutions, due to theformation of a three dimensional network among the particles, with watermolecules functioning as bridges. When prior art methods of dispersal ofparticulates in fibrous media are applied to fumed silica, thepreviously mentioned problems of fabrication, retention and porosity areobserved.

Fumed silica - albeit not in immobilized form - has been used for theremoval of hepatitis B surface antigen from fluids e.g. Stephan U.S.Pat. No. 3,686,395. Other methods of removing viruses from fluids aretaught in Porath et al U.S. Pat. No. 3,925,152, Andersson et al U.S.Pat. No. 4,168,300, Wallis et al U.S. Pat. No. 3,770,625, Vnek et alU.S. Pat. No. 3,951,937 and Bick et al U.S. Pat. No. 4,057,628. None ofthese references uses or suggests immobilized inorganic adsorbants.

At this point it is worth mentioning commonly assigned U.S. Pat. No.4,228,462 to Hou and Ostreicher for METHOD FOR REMOVING CATIONICCONTAMINANTS FROM BEVERAGES, which describes a method for preparing afilter sheet having anionic electrokinetic capture potential, whichcomprises cellulose pulp, particulate filter aids, an inorganic cationicsurface charge modifier and an inorganic anionic charge modifier,wherein the charge modifiers are cationic and anionic colloidal(inorganic) silicas respectively. Among the particulate aids arementioned diatomaceous earth, perlite, talc, silica gel, etc., having ahigh surface area, and being preferably siliceous materials such as thefiner grades of diatomaceous earth-perlite. The reason for utilizing theanionic charge modifiers, however, is so as to provide highelectrokinetic capture potential for positively charged fluidcontaminants. Further, the reason for using inorganic (rather thanorganic) charge modifiers is so as to prevent the possibility ofextracting organic elements into the filtrate. No solution is offeredfor the production of microparticulate-containing fibrous media.

A need therefore exists for a method of homogeneously immobilizingparticulate materials having millimicron sized average diameters,especially fumed silica, into fibrous matrices. A need also exists forthe products produced by such method.

SUMMARY OF THE INVENTION

It is an object of the invention to provide fibrous media containingmillimicron-sized particulates immobilized therein.

It is another object of the invention to provide a method for theproduction of fibrous media containing millimicron sized particulatesimmobilized therein.

Yet another object of the invention is to provide fibrous materialscontaining fumed silica and/or alumina immobilized therein.

Still another object of the invention is to provide a method for theproduction of fibrous media containing fumed silica dispersed therein.

Another object of the invention is to provide for a method of removal ofHBsAg from fluids by using immobilized fumed silica, and a method ofremoval of pyrogens from fluids by using immobilized alumina.

These and other objects of the invention, as will hereinafter becomemore readily apparent have been attained by providing:

A self supporting fibrous matrix, preferably a sheet, containingimmobilized therein, at least about 5% by weight of microparticulatematerial having an average diameter less than 1 micron, an organicpolycationic resin and an organic polyanionic resin, wherein said resinsare present in an amount effective to flocculate said microparticulatein said matrix.

The objects of this invention have also been attained by providing:

In a method of fabricating a self-supporting fibrous matrix containingparticulate material, the improvement wherein said material has anaverage diameter less than 1 micron and wherein said fabrication iscarried out in the presence of flocculating amounts of an organicpolycationic resin and an organic polyanionic resin.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 shows the effect of added polyanion (G-139) on the viscosity of a5% slurry of Cab-O-Sil® fumed silica in water; and shows the effect ofthe polycation (1884)/ polyanion (G-139) resin ratio on the viscositythe same slurry;

FIG. 2 shows a vacuum felting device used to measure felting times ofsilica-containing fiber slurries;

FIG. 3 shows the variation of felting time of silica-containing filtersheets containing 1% by weight polycationic resin and various amounts ofpolyanionic resin (polystyrenesulfonic acid, PSSA);

FIG. 4 shows an evaluation of delipidization of bovine serum usingvarious different types of siliceous materials in the free state; and

FIG. 5 shows a typical separation column using, as separation material,the fibrous media of the invention in the form of filter pads or discs.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is based on the discovery that fabrication ofself-supporting fibrous media containing millimicron sized particulatematerial ("micro-particulates") is greatly facilitated by addition tothe forming slurry of two types of organic polymeric resins: One typebeing a polycationic resin and the other type being a polyanionic resin.Unless both types of resins are present in the slurry of fibers andmicro-particulate material, the microparticulate material on the onehand fails to be flocculated or on the other hand, reaches a stage wherea water impermeable gel-like structure is formed, both of which preventfelting; the vacuum felting of the slurry is severely hampered, and themicro-particulates fail to be retained by the fibrous matrix. As aresult of this discovery it is possible to prepare for the first time,self supporting fibrous matrices, especially fibrous sheets, containinghigh loads of micro-particulates, without fear of loss ofmicro-particulates during production, or subsequent liquid purificationand filtration operations.

By "self-supporting fibrous media" is meant to include any coherentmatrix of fibers which will maintain its shape and form when in the drystate, i.e., will not fall apart. In its most common form theself-supporting fibrous media is a sheet made from at least onecomponent which is a long, self-bonding structural fiber, to give thesheet sufficient structural integrity in both the wet "as formed," andin the final dried condition, and also to allow handling duringprocessing and suitability for the intended end use. Cellulose fiberssuch as wood pulp, cotton, cellulose acetate or rayon can be used. Thesefibers are typically relatively large, with commercially availablediameters in the range of six to sixty micrometers. Wood pulp, can alsobe used, and has fiber diameters ranging from fifteen to twenty-fivemicrometers, and fiber lengths of about 0.85 to about 6.5 mm.

Other fibers which can be used include polyacrylonitrile fibers, nylonfibers or polyvinylchloride fibers. The preferred fibrous matrix of theinvention is a porous self-bonding sheet of cellulose fibers.

The long self-bonding structural fibers are preferably obtained fromnormally dimensioned cellulose pulp such as manila hemp, jute, caroa,sisal, bleached or unbleached kraft, kozu and the like, which typicallyhas a Canadian Standard Freeness of +400 to +800 ml. These longself-bonding fibers will constitute greater than 50% of the porousmatrix, by weight, preferably about 66-90% of the porous matrix, andmost preferably 75-83%.

When the amount of micro-particulate immobilized in the porous matrix islow, i.e., less than about 50% by weight of the media, it is preferredthat the porous matrix be formed of a self-bonding matrix of normalcellulose pulp having a Canadian Standard Freeness of +400 to +800 ml.

In the preferred embodiment of this invention it is desirable to have ahigh amount of micro-particulate, i.e., greater than about 50% by weightof the medium immobilized in the porous matrix. It is thus desirable touse the invention described in copending application U.S. Ser. No.123,467 to Hou et al filed Feb. 21, 1980, now U.S. Pat. No. 4,309,245,to maintain such high content of micro-particulate in the matrix. Theentire disclosure of this application is incorporated herein byreference. Broadly, a minor portion of cellulose pulp refined to aCanadian Standard Freeness (CSF) of between about +100 and -600 ml isincorporated with a major portion of the normally dimensioned cellulosepulp (+400 to +800 ml). In particular, from about 1% to about 10% of therefined pulp and about 10% to about 30% of the normal cellulose pulp, byweight of the medium, is contained in the matrix, the remainder beingthe particulate. Generally, the weight ratio of unrefined to highlyrefined pulp will range from about 2:1 to about 10:1, preferably 3:1 toabout 5:1. Such a mixture of pulps permits the increased retention ofthe fine particulates of the invention.

By "particulate" material is meant to include any adsorbent orparticulate used in the filter art for some time such as those disclosedin the following U.S. patents, which are herein incorporated byreference: U.S. Pat. Nos. 2,143,044; 2,746,608; 3,238,056; 3,253,978;3,591,010; 4,007,113; 4,160,059; 4,238,334; 3,669,841; 3,722,181;3,795,313; 3,983,299; 4,029,583; 3,664,967; 4,053,565 and 4,105,426.Among the useful particulate is fumed silica, fumed alumina,microcarbon, inorganic salts, and various mixtures thereof. Theparticulates include those which can serve as support for furtherchemical modification of the particles. In the present invention atleast some of the particulate material has on the average a diameter ofless than 1 micron; i.e., a Gaussian distribution of particle diameterswill have a maximum at less than 1 micron and is therefore termed"micro-particulate". Those sizes wherein this invention is most usefulare less than 100 millimicrons, most preferred less than 50millimicrons, especially between 1 and 25 millimicrons. In the preferredembodiment of the invention, the microparticulate is fumed silica orfumed alumina. As described previously, the term fumed silica includesmaterials made from the hydrolysis of SiCl₄ vapor in a flame of hydrogenand oxygen, and have diameters between 5 and 20 millimicrons. Fumedalumina includes aluminum oxide produced by flame hydrolysis ofanhydrous aluminum chloride (e.g. Aluminum Oxide C from Degussa).

Generally the micro-particulate is present in the matrix in weights ofat least about 5%, preferably 20-90% by weight, preferably about 70% bytotal weight. It is possible to use as immobilized particulate a mixtureof microparticulate (average diameter less than 1 micron), with coarserparticulate (having average diameter larger than 1 micron) in any ratio,preferably 5-95 parts by weight of micro to 95-5 parts of coarseparticulate ratio; the total particulates content would then be within5-90%, as before.

The critical aspect of the invention is the presence in the matrix ofpolysalts, formed by mixing organic polycationic resins with organicpolyanionic resins. These polysalts flocculate the microparticulates andincrease their retention in the matrix. Any organic polyanion ormixtures thereof, and polycation or mixtures thereof can be used. By"organic" is meant that the polymeric backbone is composed predominantlyfrom carbon and hydrogen atoms, with the possible presence of otherelements normally present in organic structures such as nitrogen,oxygen, sulfur and occasionally phosphorous. The common backbones arepolyvinyl, polyacrylate, polymethacrylate, polyoxy, polythioxy, and thelike. By "polymeric" is meant that the resin is composed of a pluralityof repeating units. Typical molecular weights are 2,000-1,000,000 ormore. The anionic or cationic characters of the resins are usuallyprovided by the pendant groups. For anionic resins, as an example,carboxylic acid, sulfonic acid, phosphoric acid, or phosphonic acidgroups can be used as pendant groups. For cationic resins, as anexample, the ammonium group in its mono, di, tri or tetraalkyl forms(especially lower alkyl) is the most ubiquitous pendant group, althoughany positively charged pendant group can in principle, be used(pyridinium, quinolinium, etc.) Occasionally, some resins contain thepositive charge directly along the backbone, and these can also be used.

Among the commercially available polyanionic resins which can be used,are polystyrenesulfonate (PSS, sold as Dow S21291 or Enjay RS-781);polyvinylsulfonate (made by Hercules); polyacrylic acid (sold as K702 orK714 by Goodrich); sodium polyacrylate (sold as K718); polymethylvinylether maleic anhydride copolymer (Gantrez AN-139®), or polytak RNA®(sold by Penninsular Chemicals). Among the commercially availablepolycationics which can be used are polyvinyl butyltrimethyl ammoniumchloride (sold as QT2781 or XD-7036 by Dow); polydiallyl dimethylammonium chloride (Calgon Hydroid 261); polyamine polyamide (Hercules1884®) or poly-4-vinylpyridinum chloride (Ionic PP-110®).

The variables for the fibrous media of the invention, which haveinfluence on the fabrication and final properties of the media are:

(1) Type of fiber or fibers used;

(2) Length to diameter (L/D) ratio of each type of fiber;

(3) % weight of fiber in the mixture;

(4) % weight of polysalt in the mixture and its ratio tomicroparticulates;

(5) Ratio of polyanionic to polycationic components;

(6) Type and % weight of microparticulate in the mixture;

(7) Type of resins utilized;

(8) Type of solution used for slurry (water, alcohol, solvent) to formthe matrix, and pH thereof;

(9) Slurry additives (wetting agents, impurities, etc);

(10) Conditions of microparticulate flocculation; and

(11) Felting conditions.

The more relevant of these which have not already been described supra,will be discussed as a guide, and further details can be obtained fromthe Examples. However, the adjustment of the aforementioned variables aswell as their interrelationships can be readily ascertained by thoseskilled in the art, without undue experimentation, depending on the typeof utility desired for the final media.

The total amount of polysalt (polycation+polyanion) is that sufficientor effective to flocculate the micro-particulate present in the medium,but insufficient to form an impermeable gel which would prevent slurryfelting. Normally the amount can be from 0.5 to 15% of the total weightof fibrous media, preferably 1 to 3%.

The ratio of polycation to polyanion can be adjusted so that the overallcharge of the polysalt is positive, neutral or negative, depending onthe relative proportions of one type of resin or the other. The bestresults are obtained when the polysalt is substantially neutral, i.e.,when there are stoichiometrically equivalent amounts of both resins. Theamounts can be readily determined by a simple pH titration. On the otherhand, non-stoichiometric complexes can give rise to specialty mediahaving e.g., overall positive charge (for the preferential trapping ofnegatively charged particles), or vice versa.

The self supporting fibrous matrix of the invention is preferably madeby vacuum-felting an aqueous slurry of fibers, resins and particulate.This forms a sheet having the particulate immobilized in a porousmatrix. The sheet shows a uniform high porosity, fine pore-sizestructure with excellent flow charcteristics and is substantiallyhomogeneous with respect to the fiber, resins and particulate.

The vacuum felting is performed on a foraminous surface, normally awoven wire mesh which, in practice, may vary from 50 mesh to 200 mesh,with mesh openings ranging from 280 micrometers to 70 micrometersrespectively. Finer meshes are unsuitable because of clogging problemsand/or structural inadequacy.

The size of the openings in the foraminous vacuum felting surface, andthe pore size of the cellulose fiber matrix of the formed sheet, arequite large in comparison to some or all of the dimensions of the finefiber or particulate components required to produce the desiredsubmicronic filter media sheet.

Application of polysalt to the micro-particulates can be performed inthe following different modes:

(a) Coating of microparticles in the slurry with either polycation orpolyanion, followed by flocculating the so coated microparticles withthe oppositely charged polymers and repeating, if necessary; or

(b) Adding a small amount of polycation to the microparticles in theslurry first, then followed by an equivalent amount of polyanion, andrepeating the sequence until sufficient polysalt is present in theslurry. This mode is preferred in order to add maximum amounts ofpolysalt to the system; or

(c) The least preferred mode, because of its negative effect on finalporosity is mixing the polymers first, before adding to themicroparticles to the slurry.

The sequence of adding the overall components to the slurry (i.e.,fibers, other particulates, flocculated microparticulates) appears to berelatively unimportant, provided that the slurry is subjected tocontrolled hydrodynamic shear forces during the mixing process. In the"mixed fiber" embodiment, for example, the refined pulp is added to aslurry of the unrefined pulp and then the flocculated or unflocculatedparticulate is incorporated therein. The slurry is normally prepared atabout 4% consistency and then diluted with additional water to theproper consistency required for vacuum-felting and sheet formation. Thislatter consistency will vary depending upon the type of equipment usedto form the sheet. Typically the slurry is cast onto a foraminoussurface, vacuum felted, and dried in the conventional manner. The flat,dimensionally stable sheet can be of any desired thickness and is thencut to the appropriate dimensions for each type of application.

The fibrous media of the invention containing microparticulate can beused in a myriad of applications wherever the free microparticulatewould be used, with the advantage of solid phase immobilization.

In particular, the media can be utilized in all filtrations described inaforementioned copending U.S. applications Ser. No. 147,975 filed May 8,1980; Ser. No. 164,797 filed May 30, 1980 and Ser. No. 123,467 filedFeb. 21, 1981. The media can be preferably used in the molecularseparation (e.g., chromatographic) processes disclosed in copending,application to Crowder U.S. Ser. No. 287,609 filed July 28, 1981 forMOLECULAR SEPARATION COLUMN AND USE THEREOF herein incorporated byreference. The media can also be used in preparation of zero standardserum, as that disclosed in copending U.S. application Ser. No. 276,982,filed June 24, 1981 for PROCESS FOR PREPARING A ZERO STANDARD SERUM,herein incorporated by reference. A highly preferred use of fumedsilica-containing media, is in the delipidization of biological fluids,such as for example of serum, disclosed in copending U.S. applicationSer. No. 238,686 filed Feb. 27, 1981 for TISSUE CULTURE MEDIUM, hereinincorporated by reference.

Another use for the fumed-silica containing media is in the removal oflipid enveloped viruses, for example hepatitis B surface antigen (HBsAg)from fluids, especially biological fluids, especially serum. Stillanother use is for removal of mycoplasma from fluids. Immobilized fumedalumina can be used for the removal pyrogens, endotoxins, and the like.

Because conventional techniques of flocculating particles in a fibermatrix by cationic polymers suffer from the limitation that theparticles have to be one micron or larger, vacuum felting fails to formmedia with porosity adequate for filtration. Particles of millimicronsizes such as fumed silica cannot be flocculated to form a pad, due tohydrogen bonding forces exerted between the minute particles. This isespecially so when the pH of the slurry is not neutral, causing possibleionization of the particles, as evidenced by increases in slurryviscosity.

The present invention provides a specific way to resolve these technicalproblems by adding both polycation and polyanion resins to the system.

Several advantages have been noted by the novel approach of theinvention.

(1) The proper ratio of polycation to polyanion can offset the hydrogenbonding force existing in the minute particle surfaces, enabling theparticles to be flocculated out of the slurry system;

(2) The use of mixed fibers provides fine tuning and better control inaddition to the positive-negative interaction;

(3) The discovery of this technology not only provides means ofencapsulating any small size particles into fibrous structures, but canalso speed up the production rate by cutting down on the felting time.Since the prime criteria for a high efficiency filter requires highsurface area, which can be achieved by incorporating therein particlesof relatively small size, this discovery offers a unique way of makinghigh efficiency filters;

(4) The sequential addition of polycation and polyanion results infibrous media with highly open porosity. More than 10% polysalt can beintroduced into the structure without blocking the surface reactivity ofthe adsorbent particles. The non-stoichiometric addition ofpolyelectrolytes can confer on the media either cationic or anioniccharacter, depending on the type of polyelectrolyte added in excess;

(5) The chemical nature of the polysalt complex is biocompatible andclosely resembles body tissues, such as collagen. The complex is verypermeable to water, which makes the material an ideal binder for filtersin bio-applications;

(6) The polysalt is also a superior retention aid for the fibrous media,judging from the amount of fumed silica retained in a pad structure, andas an excellent wet strength provider shown from the testing resultslater described herein.

Having now generally described this invention the same will becomebetter understood by reference to certain specific examples which areincluded herein for purposes of illustration only and are not intendedto be limiting unless otherwise specified.

EXAMPLE 1 Preparation of Cellulose Based Media Containing Fumed SilicaParticulate I. General

Large cellulosic fibers (+400 to +800 CSF) were dispersed to a 1% solidscontent in a water slurry. After complete dispersion, short fibrilatedcellulosic fiber (+40 to -10 CSF) was added to the slurry to a 3.5%consistency. This was followed by addition of the fumed silica (Aerosil380®, 7 millimicrons), anionic polymer, silicas of relatively largersizes (such as Sipernet 22®) and cationic polymer.

Sufficient agitation and mixing was allowed at every stage of addition.The mixture was pumped through a 100 mesh screen vacuum forming pot, andthe filter pad was formed upon the application of vacuum to decant thewater. The time required for the disappearance of water inside the potafter the application of vacuum is defined as the felting time. Thesmaller the particle sizes in the slurry, the longer the felting time toform the filter pad.

Table 1 shows the media made by this process, as well as the feltingtimes.

                                      TABLE 1                                     __________________________________________________________________________    Fibers                                                                                     Short                                                                              Particulate   Resin.sup.(1)                                      Large Cellu-                                                                          Micro-                                                                             Aerosil                                                                             Sipernet 22 ®.sup.(2)                                                             Polyacrylic                                                                         1884 ®                                                                        Felting time                                                                         Weight                       Filter #                                                                           losic Coho (%)                                                                        flake (%)                                                                          380 ® (%)                                                                       (%)     acid (%)                                                                            (%) (min.) (g)                          __________________________________________________________________________    1    20      10   50    20      0     2   11.5   14.7                         2    20      10   50    20      2     2   3.0    15.0                         3    20      10   50    20      4     2   4.4    15.5                         4    20      10   50    20      6     2   6.5    16.1                         5    20      10   50    20      8     2   7.5    16.6                         6    20      10   50    20      8     4   5.0    17.0                         7    20      10   50    20      0     4   20.0   15.2                         __________________________________________________________________________     .sup.(1) The amount of resin is given per 100% by weight of total fibers      and total particulate, for                                                    .sup.(2) Sipernet 22 ® is a trademark of DeGussa and represents silic     having 50-70 mμ average diameter.                                     

II. Viscosity Measurements

There is a direct relation between slurry viscosity and felting time. Itis impossible to felt the slurry when it is highly viscose. The effectof ionic resin on slurry viscosity was thus measured by Brookfieldviscosimetry, and expressed in centipoises (CPS) units. 5% of fumedsilica (Cab-0-Sil®) (Cabot Corporation) was dispersed in 350 ml ofwater.

The change of viscosity of the fumed silica slurry upon addition ofGantrez AN-139® thereto in 5% solids concentration, is shown in FIG. 1.(Gantrez AN-139® is a polymethyl vinyl ether and maleic anhydridecopolymer from GAF Corporation). The polymer is strongly anionic innature. The decrease in slurry viscosity indicates that the polymerperturbs the hydrogen bonding force between the fumed silica particles.The decrease of viscosity reached a plateau after 5 ml of Gantrezsolution had been added.

Further reduction of slurry viscosity could only be achieved by addingto the aforementioned slurry a polycationic resin, such as polycup 1884®(Hercules Chemicals), as also shown in FIG. 1. Polycup 1884® is a highmolecular weight polyamine-polyamide and epichlorohydrin copolymer,commonly used in the paper and pulp industry as a cationic flocculant.Either cationic 1884® or anionic Gantrez 139® reduced the slurryviscosity to certain level. Further addition of each only made theslurry more viscose. The combination of both, however, reduced theslurry viscosity to a minimum, which in turn reduced the requiredfelting time for filter fabrication.

III. Felting Time Measurements

Aside from the results shown in Table 1, additional measurements werecarried out. To this effect media were prepared as follows:

(a) Cellulosic fibers of different freeness were dispersed in water at1% consistency;

(b) 0.5% of cationic polymer was added to coat the fiber surfaces;

(c) Aerosil 380® (7 millimicrons) was added to the slurry to be adsorbedon the positively charged fiber surfaces through charge interaction;

(d) Sipernet 22® (50 micron) was added as mechanical means of adjustingthe media porosity;

(e) Another 0.5% of cationic polymer was added to confer the silicaparticles with a certain amount of charged sites; and

(f) Proper ratio of anionic polymers was added to the slurry as chemicalmeans of controlling the media porosity.

The felting time of the process can generally be controlled by threemechanisms:

a. The amount and the degree of refinement of the cellulosic fibers usedin the matrix structure. Highly refined fibers can retain moremicroparticles in the pad, but would result in low porosity;

b. The ratio of large silica particles (Sipernet 22) to small fumedsilica (Aerosil 380); and

c. The total amount and the ratio of polycation to polyanion.

A device for the measurement of the felting time is shown in FIG. 2,where a filter fabricating tank 1, is shown in cross section, containingslurry 2 in container 3, which is snuggly positioned by means of rubbergasket 4, and O-ring 5 on support 6 holding a 100 mesh screen, whereuponis formed the final filter. By applying vacuum through vacuum pump 7,water from the slurry is forced through the screen and drained intodrainage tank 8, where it can later be retrieved through valve 9.

The filter fabricating tank was made of a 5 inch diameter-polypropylenecylinder and a slurry containing all the components at 1% consistencywas drawn through the cylinder. The length of time required to form thewet pad after applying 25 inch vacuum through the device was recorded asthe felting time. For convenience and economics of production, thefelting time for the filter should ideally be less than 10 minutes.

A polyamine polyamide (such as polycup 1884) was used as polymericcation, and the effects of polyanion addition (such as polystyrenesulfonic acid) on the felting times of the filters are shown in Table 2and FIG. 3.

                                      TABLE 2                                     __________________________________________________________________________    Fibers              Particulate  Resin.sup.(1)                                     Large Cellu-                                                                          Short Micro-                                                                         Aerosil                                                                             Sipernet 22 ®                                                                    Polycup 1884 ®                                                                    PSSA                                                                              Felting time                                                                         Weight                    Filter #                                                                           losic Coho (%)                                                                        flake (%)                                                                            380 ® (%)                                                                       (%)    acid (%)                                                                              (%) (min.) Retained                                                                           Comments             __________________________________________________________________________     8   20      10     35    35     1.0     0.0 00          Unable to                                                                     be felted             9   20      10     35    35     1.0     0.6 8.0    90%                       10   20      10     35    35     1.0     1.0 1.0    91%                       11   20      10     35    35     1.0     3.0 5.4    97%                       12   20      10     35    35     1.0     4.0 10.0   99%                       13   20      10     35    35     3.0     3.0 3.0                              14   20      10     35    35     6.0     6.0 6.0                              15   20      10     30    40     1.0     1.0 3.0                              16   20      10     25    45     1.0     1.0 7.0                              17   20      10     20    50     1.0     1.0 10.0                             18   20      10     35    35     1.0     2.0.sup.(2)                                                                       2.5                              __________________________________________________________________________     .sup.(1) The amount of resin is given per 100% by weight of total fibers      and total particulate, for                                                    .sup.(2) Polyacrylic Acid instead of PSSA.                               

It can be seen that the best mode of felting is with formulations havingequal amounts of anion to cation (cf. filter 10). The effect of totalpolysalt (at a 1:1 ratio) on felting times is shown by comparing filters10 with 13 and 14, and shows that smaller total amounts of polysaltfavor shorter times. Finally, the relation of amount of Aerosil 380 tofelting time can be ascertained by comparing filters 10 with filter 15through 17.

EXAMPLE 2 Delipidization of Serum Using the Media of the Invention I.Preliminary Evaluation of Free Silicas on Lipid Adsorptivity

To ascertain what type of silica would be the best for lipid adsorption,the adsorption isotherms of lipids in bovine serum by various types ofsilica were studied, with the results tabulated in Table 3 and plottedin FIG. 4. The Freundlich isotherm was adopted by plotting (X/M) inmilligrams of lipids per deciliter of serum adsorbed by one gram ofsilica vs. (C) the concentration of unadsorbed lipids remaining in theserum. The slope of the line gives the rate of change of the equilibriumrelation between adsorption and lipid concentration. The amount of lipidadsorbed by unit weight of specific types of silica is a direct measureof its adsorptive capacity, which in turn gives its available number ofactive sites for lipid adsorption.

1. Method of Lipid Determination

The amount of lipid in bovine serum was determined by the sulfo-phosphovanillin method by measuring the (pink) color developed at a wavelengthof 540 nm, with olive oil serving as a standard. The Procedure was asfollows:

(1) 10 μL of H₂ O (blank), standard (standard), or serum (unknown), wereadded to 10 mL test tubes;

(2) 0.1 mL conc. H₂ SO₄ was added to each tube and mixed;

(3) 11 tubes were placed in boiling H₂ O for 10 min. (±1 min.) thencooled in cold H₂ O for ≃ 5 min;

(4) 5 mL of the phospho-vanillin reagent was added to each tube, mixedand incubated at 37° C. (±2° C.) in a H₂ O bath for 15 min;

(5) The tubes were cooled for ≃ 5 min. and the A₅₄₀ was measured within30 min. (Instrument was set on blank).

The reagents were as follows:

(1) Vanillin reagent: 6 g/L H₂ O (1 L vol. flask) (stable for 2 mo. whenstored in brown bottle at room temperature);

(2) Phospho-vanillin reagent: 350 mL of vanillin reagent and 50 mL H₂ Oto a 2 L Erlenmeyer. Add 600 mL conc. phosphoric acid with constantstirring. Store in brown bottle at room temperature;

(3) Standard: 600 mg olive oil to 100 mL vol. flask. Bring to vol. withabsolute ethanol. Store at 4°-7° C.

To carry out the adsorption study, the following procedure wasperformed: The specific amount of silica was measured accurately andadded at the vortex point of 1 mL bovine serum under agitation, the tubewas then incubated at 37° C. for 1 hour and centrifuged at 3000 rpm for10 minutes to separate the adsorbent from the serum. The amount of lipidleft in the serum supernatant was determined with aid of the resultstabulated in Table 3.

                                      TABLE 3                                     __________________________________________________________________________    Lipid Removal from Bovine Serum by Various types of Silica                     Nature of Silica                                                                            M Weight of                                                                           C Concentration                                                                       X Concentration                                                                        %                                                                                  ##STR1##                                Product Silica Applied                                                                        of Unadsorbed                                                                         of Adsorbed                                                                           Lipid                                                                              (mg/dL) of Lipid                  Manufacturer                                                                         (size)    (in grams)                                                                          Lipid (mg/dL)                                                                         Lipid (mg/dL)                                                                         Removed                                                                            Adsorbed/g of                     __________________________________________________________________________                                                silica                            Davidson                                                                             952       0        261     0                                           Chemical                                                                             silica gel                                                                              0.15     251     10   4     66                                      (50 micron)                                                                             0.35     241     20   8     57                                                0.50     235     26   10    52                               Decalite                                                                             Perlite   0        306                                                        416       0.15     303    No effect   20                                      (15 micron)                                                                             0.35     300                17                                                0.50     298                16                               Davidson                                                                             Syloid    0        306     0                                                  266       0.15     246     60   19.6 400                                      (2 micron)                                                                              0.35     200    106   34.7 302                                                0.50     151    155   50.6 310                               DeGussa                                                                              Aerosil   0        286                                                        380       0.15     166    120   58.0 800                                      (7 millimicron)                                                                         0.35      73    213   74.4 608                                                0.50      44    242   84.6 484                               DeGussa                                                                              Sipernet 22S                                                                            0        296                                                        (18 millimicron)                                                                        0.15     188    108   36.4 720                                                0.35      98    198   66.9 565                                                0.50      66    230   77.7 460                               Cabot  M-5       0        290                                                        (14 millimicron)                                                                        0.15     200     90   31.0 600                                                0.35     130    160   55.2 457                                                0.50      59    231   79.6 462                               __________________________________________________________________________

The results indicate that natural silicas such as diatomaceous earth orperlite show no, or very little, adsorption of lipids. Silica gel isslightly better than the natural silicates. Fumed silica is far betterthan silica made from solution precipitation processes.

In attempting to carry out batchwise processes with fumed silica onequickly runs into serious problems. Even a small amount of fumed silicapacked in a column will create extremely high pressure upon contact withbuffer solutions due to the formation of a three dimensional networkamong the silica particles, with water molecules functioning as bridges.

Using the silica containing media of the invention, however, it ispossible to overcome all of these difficulties as shown in the followingstudy of delipidization of human serum.

II. Lipid Removal from Human Serum

Unfiltered human serum having a lipid content of 500 mg/dl wascontinuously filtered through a plurality of filters 10 and a pluralityof filters 18 (supra), and the reduction in lipid concentration is shownin Table 4, with reduction expressed as % lipid removal.

                  TABLE 4                                                         ______________________________________                                                    No.                                                                           of     Filter                                                                              Flow        Vol.  %                                  Serum Fil-  fil-   Weight                                                                              Rate   Δ P                                                                          filtered                                                                            effective                          Sample                                                                              ter   ters   (g)   (ml/min)                                                                             (psi)                                                                              (ml)  Removal                            ______________________________________                                        1     10    15     7.2   1.6    10   30    87                                                                      15    75                                                                      75                                                                            20    170                                                                     60                                                                            25    300                                                                     40                                       2     18     8     6.2   0.6    20   20    60                                                                      26    50                                                                      45                                                                            30    75                                                                      29                                       ______________________________________                                    

The results indicate that large volumes of serum can quickly andefficiently be delipidized using the filters of the present invention.

EXAMPLE 3 Pyrogen Removal by Fumed Alumina Filter I. Fumed Alumina

Fumed alumina with the same particle sizes as fumed silica can beproduced by flame hydrolysis of anhydrous aluminum chloride An exampleof such product is aluminum oxide C produced by Degussa.

II Pyrogen Removal by Fumed Alumina Filter

Pyrogens are substances which, when injected into animals, produce feverand other reactions which can result in death. The most common pyrogenicsubstances are endotoxins, the lipopolysaccharide (LPS) components ofgram-negative bacterial organisms. Endotoxins are pyrogens capable ofproducing febrile reactions in the human body after intravenous doses ofas small as 1 ng/kg. The principal method for keeping parenterals freeof contaminating bacterial endotoxins is to keep the manufacturingprocess and all subsequent handling sterile. Because maintainingsterility is difficult in many production processes, a reliable methodis needed for removal of endotoxin from an accidentally contaminatedproduct. Asbestos fiber beds can be used to remove pyrogens but theiruse in the United States is prohibited by Food and Drug Administrationregulations.

To evaluate the performance of fumed alumina filters two types oftesting procedures were used. The first involved the passage of smallvolumes of test solution (10-50 mL) through 0.9 cm² disc filters. Thetest solution was passed through the filters with the aid of a syringeat a constant flow rate. In large volume experiments usually ten litersof test solution was passed through 3.9 cm² filters at a flow rate of3.05 gal./sq.ft./min. For contamination of the test solutions E. coliLPS (obtained from Sigma Chemical Company) was used. Limulus AmebocyteLysate (LAL) for endotoxin determination was obtained from M.A.Bioproducts. Its sensitivity for endotoxin determination was evaluatedfor each test solution and was found to be 30 pg/mL for almost all ofthe test solution.

To determine endotoxin concentration, 0.1 mL lysate was mixed with 0.1of test sample and allowed to incubate in a small test tube placed in awater bath at 37° C. for 1 hr. The formation of a gel that did notcollapse upon two 180" inversions of the tube indicated a positive test.A titration of the endotoxin was performed in every challenge tovalidate the sensitivity and efficiency of the LAL test.

All test samples were assayed both undiluted and at dilutions of 1:2,1:4, 1:8, 1:16, 1:32 and 1:64, etc. The end point of each test wasreached when the specimen sample showed no formation of a firm clot.This endpoint dilution was then multiplied by the sensitivity of the LALto give the approximate value of endotoxin present in the sample. Thus,concentrations of endotoxins in these studies are reported as multiplesof end-point dilutions in picograms per millilter.

Procedure

A 0.9% NaCl pH 6.7 solution for injection was contaminated with theconcentration of E. coli endotoxin per mL indicated in Table 5 andfiltered through 13 mm dia. (0.9 cm² surface area) unautoclaved filters.All filters were prewashed with 30 mL of test solution.

A filter applied for pyrogen removal is fabricated based on theformulation 10 listed in Table 2, by replacing Aerosil 380 with fumedalumina. The filter was found capable of reducing endotoxin levels toless than the limits of the assay (30 pg/mL) from small volumes when theconcentration of endotoxin ranged from 1,000 to 1,000,000 pg/mL. Resultsare shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        Effect on Endotoxin Concentration                                             Removal by Fumed Alumina Filter                                                                Endotoxin Concentration                                                       in Filtrate (pg/mL)                                          Concentration of Volume passed through Filter                                 Endotoxin (pg/mL)                                                                              10 mL      50 mL                                             ______________________________________                                           1,000         <30        <30                                                 100,000        <30        <30                                               1,000,000        <30        <30                                               ______________________________________                                    

EXAMPLE 4 Mycoplasma Removal by Fumed Silica Filter

Contamination of cell cultures by mycoplasma is a common problem intissue culture. The fumed silica filter of the invention can removemycoplasma very effectively, through an adsorption mechanism.

I. Test method

Detection of mycoplasma was performed by using a kit purchased from theFlow Laboratories, Inc., McLean, Vir.

(a) Inoculation

10 mL of bovine serum were mixed with 40 mL broth as liquid medium. Thepositive controls were resuspended in 10 mL broth as standard. Theliquid medium was inoculated on agar plates in 72 hours or when signs ofgrowth appeared.

(b) Incubation

Agar plates were incubated in an inverted position in an atmosphere of5% CO₂ in N₂ at 35° to 37° C. Additional plates may be incubatedaerobically. The liquid medium was sealed and incubated aerobically.There is some question as to the usefulness of aerobic incubation.Certainly most mycoplasmas are isolated under low oxygen and increasedCO₂ concentration.

(c) Examination of Mycoplasma Cultures

Mycoplasma colonies will frequently develop within four days ofincubation but all agar plates were incubated two-three weeks beforebeing discarded as negative.

II. Filtration procedure

Filter 10 was cut into 25 mm diameter discs, and stacked in a plasticcolumn with 10 filters. The bovine serum was pumped through the columnat 1 mL/min. of flow with a perastaltic pump.

The filtrates were collected in the fractionation collector andinoculated on agar plates to detect mycoplasma by the Flow Lab kit asdescribed above. The mycoplasma plaque assay was counted after two weeksof inoculation. The percentage of plaque reduction is reported as theefficiency on mycoplasma removal by the filter, in Table 6:

                  TABLE 6                                                         ______________________________________                                              Volume                                                                        of Serum Filtration                                                                             Mean Number %                                         Filter                                                                              filtered Rate     of Plaques  Plaque                                    No.   (mL)     (mL/min) Influent                                                                             Effluent                                                                             Reduction                               ______________________________________                                        10    10       1.0      1170    8     99.3                                          20       "        "      10     99.1                                          30       "        "      28     97.6                                          40       "        "      38     96.7                                    10    10       5.0      550    80     85.5                                          20       "        "      80     "                                             30       "        "      100    82.0                                          40       "        "      110    81.8                                          50       "        "      140    75.0                                    ______________________________________                                    

EXAMPLE 5 Removal of Viral Hepatitis by Fumed Silica Filters

The phenomena of removal of lipids by a fumed silica filter of theinvention can also be applied to the removal of virus with lipidenvelopes. An example of such is the hepatitis surface antigen in humanserum.

The use of fumed silica "Aerosil"® for selective adsorption oflipoprotein from human sera has been investigated by Stephan, supra, whohas recommended such treatment for the improvement of storage stabilityof sera in infusion. HBsAg behaves like normal serum lipoproteins andcan be adsorbed very effectively on free Aerosil®. Such treatment,however, can only be performed in small volumes of sera, since itinvolves high speed centrifugation to precipitate out the Aerosil®particles.

It is also a nuisance to try to remove the traces amount of Aerosil®left in the supernatant. A filter containing fumed silica for removingHBsAg and lipids makes the process applicable for large volumefiltration with minimum protein loss.

I. HBsAg Determinations

HBsAg determinations were performed by Procedure B of Austria II-125 ofAbbott Laboratories; a solid phase radioimmunoassay technique to measureHBsAg levels in serum. (See brochure of Abbott Laboratories, DiagnosticsDivision, entitled "Antibody to Hepatitis B Surface Antigen ¹²⁵ I(Human) Ausria® II-125".) Polystyrene beads coated with guinea pigantibody are supplied in the kit. Patient serum is added and, duringincubation HBsAg, if present, is fixed to the antibody. When antibodytagged with ¹²⁵ I is added, it binds to any HBsAg on the beads creatingan antibody-antigen-antibody sandwich.

Within limits, the greater the amount of antigen in the serum specimen,the higher the final count rate.

II. Preparation of Serum Samples

Plasma from chronic carriers of HBsAg was obtained from Herman Hospitalin Houston, Tex. The patients' plasma was converted to serum by calciumchloride precipitation to remove fibrinogen. Most of the serum samplesshowed HBsAg over 100 counts per minute which is over the sensitivityrange of the Abbott Ausria kit. Dilution with 0.05 M salt phosphatebuffer pH 7.2 in the range of 1:5 to 1:10 ratio will bring the countsdown to the accuracy region detectable by Abbott kit. It is our generalpractice to keep the counts around 50-60,000 CPM, with correspondingHBsAg concentration around 0.2-0.3 microgram per mL of serum. Thediluted serum was stored in a freezer to avoid the decrease of HBsAgthrough precipitation during the time of storage.

III. Preparation of Filter Pad

Filter No. 10 was used for the test.

IV. Experimental Set-Up

A schematic diagram of a column for serum filtration is shown in FIG. 5.A peristaltic pump capable of handling ΔP up to 60 PSI was used. Tenfilters of size 25 mm were stacked in the column. The test results wereas follows:

                  TABLE 7                                                         ______________________________________                                                        Volume               %                                              Flow      of Serum Counts per Min.                                                                           Removal                                  Filter                                                                              Rate      filtered (CPM)       based                                    Type  (mL/min)  (mL)     Influent                                                                             Effluent                                                                             on CPM                                 ______________________________________                                        10    2 mL/min  10       2284    175   92.7                                                   20       "       169   92.6                                                   30       "       170   92.6                                   10    2 mL/min  25       7818   1028   87.0                                                   50       "      3912   50.0                                                   75       "      5845   25.0                                                   100      "      6659   15.0                                   ______________________________________                                    

Having now fully described this invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit or scope of the inventionset forth herein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A self supporting fibrous matrix containingimmobilized therein at least about 5% by weight of microparticulatematerial with an average diameter of less than 1 micron, an organicpolycationic resin and an organic polyanionic resin, wherein said resinsare present in an amount effective to flocculate said microparticulatematerial in said matrix, wherein said microparticulate material isdistributed substantially uniformly throughout a cross-section of saidmatrix.
 2. The matrix of claim 1 wherein, said microparticulate materialhas an average diameter less than 100 millimicrons.
 3. The matrix ofclaim 2 wherein, said average diameter is between 1 and 25 millimicrons.4. The matrix of claim 1 which contains, in addition, coarse particulatehaving an average diameter greater than 1 micron.
 5. The matrix ofclaims 1 or 4 wherein, the total particulate weight is 5-90% by weightof the matrix.
 6. The matrix of claim 5 wherein, the total particulateweight is about 70% of the matrix.
 7. The matrix of claim 4 whichcontains 5-95 parts of micro-particulate to 95-9 parts of coarseparticulate, per 100 parts by weight of total particulate.
 8. The matrixof claims 1 or 4 wherein, said microparticulate is present as 5-90% byweight of the matrix.
 9. The matrix of claim 1 which comprises at leastone component which is a long self-bonding structural fiber.
 10. Thematrix of claim 9, wherein said fiber is unrefined cellulose pulp havinga Canadian Standard Freeness of +400 to +800 ml.
 11. The matrix of claim10 which also comprises highly refined cellulose pulp with a CanadianStandard Freeness of between +100 to -600 ml.
 12. The matrix of claim11, wherein the weight ratio of unrefined pulp to highly refined pulp isabout 2:1 to about 10:1.
 13. The matrix of claim 12, wherein the ratiois 3:1 to about 5:1.
 14. The matrix of claim 1, wherein said organicpolyanionic resin carries pendant groups selected from the groupconsisting of carboxyl, sulfonyl, phosphoryl, phosphonyl and mixturesthereof.
 15. The matrix of claim 14, wherein said anionic resin ispolyacrylic acid, polymethacrylic acid, polystyrenesulfonic acid,polyvinylsulfonic acid, or polymethylvinyl ether - maleic anhydridecopolymer.
 16. The matrix of claim 1 wherein said organic polycationicresin carries positively charged pendant groups or a positively chargedbackbone.
 17. The matrix of claim 1, wherein the total amount ofpolyanionic plus polycationic resin is 0.5-15% by weight of the matrix.18. The matrix of claim 1, wherein the ratio of polycationic topolyanionic resin results in essential neutralization of each resin. 19.The matrix of claim 1, wherein said microparticulate is fumed silica.20. The matrix of claim 19, wherein said fumed silica has an averageparticle diameter of 5 to 20 millimicrons.
 21. The matrix of claim 19which also comprises coarse siliceous particulate having an averagediameter greater than 1 micron.
 22. The matrix of any of claims 19 or21, wherein the total particulate weight is 10-90% of the matrix. 23.The matrix of claim 22 which contains 5-95 parts of fumed silica to 95-5parts of coarser siliceous particulate by 100 parts by weight of totalparticulate.
 24. The matrix of claims 19 or 21 which comprises onefibrous component which is unrefined cellulose pulp having a CSF of +400to +800 ml; and another fibrous component which is highly refinedcellulose pulp having a CSF of +100 to -600 ml.
 25. The matrix of claim24, wherein the ratio of unrefined to refined pulp is about 2:1 to about10:1.
 26. The matrix of claim 1 wherein said microparticulate is fumedalumina.
 27. The matrix of any of claim 19 or 26 which is in the form ofa sheet.
 28. A separation column containing the self-supporting fibrousmatrix of claim
 1. 29. The column of claim 28 wherein said fibrousmatrix comprises a plurality of sheets.
 30. The column of claim 29wherein said sheets are in the form of discs.
 31. In a process offabricating a self-supporting fibrous matrix containing immobilizedtherein particulate material, the improvement wherein said material isat least about 5% by weight of a microparticulate material having anaverage diameter less than 1 micron, and wherein said fabrication iscarried out by forming a slurry containing fibrous material, saidmicroparticulate material and flocculating amounts of an organicpolycationic resin and an organic polyanionic resin; andvacuum feltingsaid slurry to thereby form said self-supporting fibrous matrix.
 32. Theprocess of claim 21 whrein said microparticulate has an average diameterless than 100 millimicrons.
 33. The process of claim 32 wherein saidaverage diameter is between 1 and 25 millimicrons.
 34. The process ofclaim 31 wherein said matrix contains, in addition, coarse particulatehaving an average diameter greater than 1 micron.
 35. The process ofclaims 31 or 34 wherein the total particulate weight is 5-90% by weightof the matrix.
 36. The process of claim 31 wherein said matrix comprisesat least one component which is a long selfbonding structural fiber. 37.The process of claim 36 wherein said fiber is unrefined cellulose pulphaving a Canadian Standard Freeness of +400 to +800 ml.
 38. The processof claim 37 wherein said matrix also comprises highly refined cellulosepulp with a Canadian Standard Freeness of between +100 to -600 ml. 39.The process of claim 38 wherein the weight ratio of unrefined pulp tohighly refined pulp is about 2:1 to about 10:1.
 40. The process of claim31 wherein said organic polyanionic resin carries pendant groupsselected from the group consisting of carboxyl, sulfonyl, phosphoryl,phosphonyl and mixtures thereof.
 41. The process of claim 31 whereinsaid organic polycationic resin carries positively charged pendantgroups or a positively charged backbone.
 42. The process of claim 31wherein, the total amount of polyanionic plus polycationic resin is0.5-15% by weight of the matrix.
 43. The process of claim 31 wherein theratio of polycationic to polyanionic resin results in essentialneutralization of each resin.
 44. The process of claim 31, wherein saidmicroparticulate is fumed silica.
 45. The process of claim 44 whereinsaid fumed silica has an average particle diameter of 5 to 20millimicrons.
 46. The process of claim 44, wherein said matrix alsocomprises coarse siliceous particulate having an average diametergreater than 1 micron.
 47. The process of claim 31 whichcomprisesforming a slurry containing said micro-particulate, adding tosaid slurry either one of said polycationic or polyanionic resins, thenadding to said slurry the other of said polycationic or polyanionicresins, thereby flocculating said micro-particulate in said slurry. 48.The process of claim 31, which comprises:(1) forming a slurry containingsaid microparticulate, (2) adding to said slurry less than the totalflocculating amount of either one of said polycationic or polyanionicresins, then (3) adding to said slurry less than the total amount of theother of said polycationic or polyanionic resins, then (4) repeatingsaid sequence of steps (2) and (3) until said micro-particulate isflocculated in said slurry.